Test unit for telephone set - has same shape as handset with dial, display and audio test-tone generator

摘要

The test unit has an audio generator and a loudspeaker to produce a test tone and a microphone. The generator produces a rectangular wave output. A push-button earths the unit. The unit also has a dial, an input push-pull circuit and a display. The display consists of a l.e.d. that shows when a voltage has been applied to the line. Alternatively, the display may be a micro-ammeter. The generator's frequency is c.800Hz. The circuit's casing is shaped like that of a telephone handset and has the dial in the stem between the microphone and loudspeaker capsules.